When a pure tone is produced, the sound radiates outward. As it does, different areas (of air) are in rarefaction and compression. Wavelength is the physical distance, generally measured in feet, between areas where the sound wave is in the same phase of vibration. For example, if air molecules are in maximal compression at one place and then are rarefied one foot later, then maximally compressed again two feet away from that original compression place, then the wavelength is 2 feet. Wavelength is related to frequency and computed by dividing the speed of sound by the frequency.